p75 nerve growth factor receptor modulates p140trkA kinase activity, but not ligand internalization, in PC12 cells

Abstract

The biological activity of nerve growth factor (NGF) has been shown to be mediated by the p140^trkA receptor tyrosine kinase, while the role of the p75 NGF receptor (p75^NGFR) is still unresolved. Here we have investigated the relative contribution of p140^trkA and p75^NGFR to early consequences of NGF binding: ligand internalization, p140^trkA autophosphorylation, and tyrosine phosphorylation of Shc, phospholipase C_γ‐1 (PLC_γ‐1), and extracellular signal‐regulated kinases (ERKs). It was found that NGF internalization was neither prevented by blocking p140^trkA activity using the protein kinase inhibitors methylthioadenosine, staurosporine, and K‐252a, nor by inhibiting NGF binding to p75^NGFR with antibodies. However, when NGF binding to p140^trkA was reduced by the use of a synthetic peptide corresponding to amino acids 36–53 of human p140^trkA, internalization of NGF was decreased. Thus, at least in PC12 cells, internalization appears to require binding of NGF to p140^trkA, but occurs irrespective of p140^trkA kinase activity and ligand occupancy of p75^NGFR. The NGF triple mutant Lys‐32/Lys‐34/Glu‐35 to Ala, which has been demonstrated to bind to p140^trkA, but not to p75^NGFR, induced tyrosine phosphorylation more rapidly than wild‐type NGF. Likewise, NGF‐induced tyrosine phosphorylation was accelerated when NGF binding to p75^NGFR was prevented with REX‐IgG. These findings indicate that NGF binding by p75^NGFR may modulate NGF‐induced p140^trkA kinase activity.